You have 20mL of a solution of concentration 0.015mg/mL of x in test tube #1.
Prepare serial dilutions of your stock solution, and calculated the concentration of x as follows:

Add 10mL of distilled water and 10mL of stock solution from test tube 1 to test tube 2.

Add 10mL of distilled water and 10mL of stock solution from test tube 2 to test tube 3.

.....and it keeps going.

So, obviously I will use the formula c1v1=c2v2.
So the concentration of test tube #1 is just 0.015mg/mL.
So, for test tube #2, c1=0.015mg/mL, and v1=20mL, and c2 is what we are trying to solve for. But what is v2? Can someone please tell me what v2 is and explain what it is. I think it's 40mL, giving c2=0.00750 mg/mL. But can someone please explain how I know what volumes to add! And maybe even explain what v2 for the next test tube would be? Is it 80mL?